Electrochromism is the change in light-absorbing properties of a material under the influence of an applied voltage. The induced coloration will remain even after the voltage is removed. An electrochromic material has the property of changing color when the voltage is applied across the material or, alternatively, if a current is passed through it. The electrochromic material can be made to return to its original light absorbing state or color by reversing the polarity of the voltage or current. By changing the polarity of the applied voltage or current, known electrochromic materials which may be both organic and inorganic in nature can be cycled in such a manner so that the color change may be made from clear or transparent to colored, or from one color state to another color state, the last case being characteristic but not exclusive of organic materials. For example, electrochromic displays may go from clear to blue, from yellow to blue, from red to blue, from green to blue/black, etc., and the reverse. This is in contradistinction to other displays such as light-emitting diode displays which may go from off to red or liquid crystal displays which may go from clear to blue and reverse. Once an electrochromic display is in either its color or transparent state, it will remain in such a state for a considerable lengthy period of time, even though the power has been turned off. This is also in contradistinction to light-emitting diode displays and liquid crystal displays which require continuous power in order to be seen. The characteristics which are possessed by electrochromic displays include a low voltage operation, low power requirements, storage of the display without the dissipation of power, potentially low cost as well as a relatively simple construction and will provide a pleasing display in that the color will be present in a relatively good contrast compared to the background while providing a wide angle of viewing.
The electrochromic material which is utilized in electrochromic displays must possess a color center or other color absorbing structure having some optical absorption in the visible light range plus the presence of both electronic and ionic conduction. One class of electrochromic materials of the type hereinafter set forth in greater detail will possess high ion mobilities and the ability to produce a strong optical absorption in the visible light range upon injection of electrons. The protons which are present in the electrolyte which is in contact with the electrochromic material will contact said electrochromic material and provide a means of maintaining charge neutrality in the electrochromic material. An example of this color change occurs when an electrochromic material such as tungsten trioxide is subJected to the action of an applied voltage through an electrolyte to form what is called tungsten bronze, said tungsten bronze producing a blue color from the colorless tungsten trioxide. To reverse the coloration process and bleach out the blue color produced by the tungsten bronze, the polarity is reversed so that the electrons and protons leave the electrochromic material, said polarity reversal being effected until the entire tungsten bronze has been reconverted to the tungsten trioxide and the latter is restored to its original colorless state.
In the past, there have been electrochromic devices utilizing this phenomena. For example, U.S. Pat. No. 4,306,773 discloses an electrochromic display device utilizing a tungsten trioxide film and an aqueous acidic electrolyte in contact with the surface of said film. Likewise, U.S. Pat. No. 3,843,232 also discloses an electrochromic device utilizing an electrochromic material and an ion-conductive medium between the electrochromic material such as tungsten trioxide and a counterelectrode such as palladium, said ion-conductive medium being liquid in nature such as a strong sulfuric acid solution. It is to be noted that both of these patents disclose the use of a liquid electrolyte such as sulfuric acid. The use of liquid electrolytes possess certain disadvantages. For example, the display cell utilizing a liquid electrolyte requires a relatively complicated construction inasmuch as care must be taken to insure a permanent seal of the electrolyte within the cell inasmuch as any leakage of the electrolyte will result in a breakdown of the display device. The care and means which are undertaken to insure the permanent seal of the electrolyte will, of necessity, add to the expense in manufacturing such a device. Likewise, the acid may have a tendency to attack the electrochromic material, thus leading to a failure of the device after a period of time.
In addition to the aforementioned U.S. patents, other U.S. patents disclose solid electrochromic display devices. In this respect, U.S. Pat. No. 3,995,943 shows a display device utilizing as an electrochromic material an oxide of tungsten or vanadium and a solid electrolyte comprising a mixed inorganic silver salt, one salt being silver iodide. U.S. Pat. No. 4,306,774 utilizes, as the electrochromic material a layer of material being selected from the group consisting of a diphthalocyanine of a rare earth element, yttrium and scandium. The solid electrolyte which is employed is selected from the group consisting of a heteropoly acid and hydrogen uranyl phosphate. Another solid state electrochromic device is shown in U.S. Pat. No. 4,350,414 which comprises a pair of electrodes, an oxidizable film and a reducible film capable of a redox reaction as well as an insulating film positioned between the oxidizable film and the insulating film. U.S. Pat. No. 4,184,751 discloses an electrochromic chromic display utilizing a metal diphthalocyanine complex and as the electrolyte a porous solid which has been saturated with an inert electrolyte such as an aqueous solution of potassium chloride. The porous solid which is illustrated in this patent comprises calcium sulfate dihydride, commonly known as plaster of paris which is white in nature and is preferred in order that the plaster will not adversely affect the colors which are generated during the display. However, such a system will still involve the use of a wet electrolyte with the attendant costs and disadvantages previously discussed with reference to the use of liquid electrolytes.
In contrast to these discussed electrochromic devices, we have now discovered that an electrochromic device may be prepared in a solid state utilizing, as a solid electrolyte, a blend of an organic polymer and a heteropoly acid or salt thereof. This solid electrolyte will provide the advantage of being an excellent proton-conducting material whereby the disassociated molecular hydrogen will migrate through the polymer as a proton and provide the impetus necessary to effect the desired color change. In addition, the polymer blend electrolyte will also possess other advantages over the electrolytes of prior use, said advantages being subsequently more fully discussed.